1. Field of the Invention
This invention relates to digital-to-analog converters (DACs). More particularly, this invention relates to monolithic converters especially suitable for use as part of a single-chip analog I/O system, providing a complete interface between a microprocessor and the analog world. This invention provides improved converter performance including faster speed and reduced errors.
2. Description of the Prior Art
A wide variety of DACs have been proposed over the years, and some have gone into extensive commercial use. One early converter using discrete transistors is described in U.S. Pat. No. 3,685,045 (Pastoriza) which shows PNP current source transistors driving an R/2R ladder network; the transistor current sources are turned on or off by a code-controlled circuit connected to each transistor emitter. U.S. Pat. No. 3,961,326 (Craven) shows an IC converter using a bipolar differential switch pair for steering the current of a current source transistor to either a summing bus or a ground line; the emitter areas of the current source transistors are proportioned to the magnitude of the respective bit current. U.S. Pat. No. 4,323,795 (Holloway, et al) shows a monolithic DAC somewhat similar in design to the discrete converter of the '045 patent referred to above; PNP transistor current sources directly drive an R/2R ladder and are turned on or off by circuitry connected to the transistor emitters.
Although the converters shown in the above-mentioned prior art patents have performed well in many circumstances, their capabilities have been found lacking in important respects when used in more modern applications, particularly when used as part of a high-speed interface for a microprocessor-based data acquisition system. Accordingly, it is a principal object of this invention to provide a digital-to-analog converter having significant improvements over prior art designs particularly with respect to operational characteristics such as speed and error rate.